Memory Barrier
A processor/runtime ordering primitive that constrains how memory operations may be observed across execution contexts.
Ordering, Not Mutual Exclusion
Modern CPUs and compilers can reorder memory operations when single-thread semantics are preserved. A memory barrier constrains selected reorderings so that other execution contexts observe required ordering relationships.
A barrier does not by itself protect a compound invariant like a mutex does.
Language vs Hardware
High-level atomics and synchronization constructs map to language memory-model guarantees, which the runtime/compiler then implements using appropriate machine instructions. Writing architecture-specific fences directly is therefore rarely the first choice in managed code.